Some observations on the biochemistry and inhibition of nitrification

The effects of compounds potentially toxic to nitrification were examined using a laboratory procedure which involved aerating activated sludge mixed liquors in the presence of measured amounts of these compounds. Results for a variety of compound types are given and the applicability of the procedure discussed. The results obtained for thiourea and its analogues provide further support for an hypothesis suggested as an explanation of the toxicity of compounds containing the NCS group; this is based on a model reaction involving copper in oxidases in Nitrosomonas. It is also suggested that the losses in inorganic nitrogen observed during nitrification (up to 43% and which are lowered as the extent of nitrification is lowered) might be due to a biochemical mechanism in which nitrous oxide is produced by Nitrosomonas from nitrite using a hydroxylamine-nitrite reductase. Recommendations indicating further laboratory and field studies are made.